In any network in which devices share a single transmission channel (e.g., a wireless network, a shared bus network, etc.), every device can hear all or most of the other devices. In such a network when two devices transmit a signal at the same time, the two transmitted signals are considered to have collided, which can cause those signals to become unintelligible, meaning no data is sent, and thus the use of the transmission medium (i.e., the transmission channel) is lost for a time. Both signals will then have to be resent, lowering the data throughput of the network. If this happens often enough, the entire transmission medium could be rendered ineffective. It's therefore typically desirable to avoid, or at least minimize, such collisions.
One way to accomplish this is to accept the existence of collisions and try and minimize their occurrence or their effect through the use of a collision-based protocol. A collision-based protocol allows multiple devices to transmit data when they need to without any prearrangement. For example, an ALOHA protocol allows each device to transmit data when it wishes without monitoring the channel at all. In this protocol, fatal collisions simply prompt a retransmission. This results in comparatively higher collision rates and reduced efficiency as more transmissions must be resent. A slotted ALOHA protocol further refines the idea by dividing the available transmission time up into discrete time slots. It uses an ALOHA protocol within those time slots, but only allows each device to start a transmission at the beginning of one of the time slots. Thus, if no collision occurs immediately, it won't happen at all. And a carrier sense multiple access (CSMA) protocol requires a potential transmitting device to listen to the transmission channel prior to transmitting, and only allows it to transmit if it hears no one else transmitting (i.e., only if the transmission channel is clear).
Another way to reduce or eliminate collisions is to use a time division multiple access (TDMA) protocol, in which a network coordinator (or master device) restricts the use of the channel to one network device (or slave device) at any given time. A time slotting TDMA protocol divides the available transmission time up into discrete time slots and assigns them to individual devices or device pairs. Each device can only transmit in its allotted time, but knows that it won't have a collision with another device. A polling TDMA protocol does not assign specific time slots to each device, but rather has a network coordinator periodically poll each device in the network to see if it has data to send. The network coordinator can then assign channel time to each device based on its response to the poll.
A polling protocol allows great flexibility in allocating the available channel time. However, it incurs an overhead cost in that the network coordinator must repeatedly poll each device in the network to see if they have data to send. This can be particularly wasteful if no device in the network has any data to send. In such a case, the network coordinator will continually transmit polling signals, and the devices will repeatedly have to respond, when neither side has anything new to say.
A polling protocol can therefore create unnecessary signal traffic on a limited transmission medium that may well be shared with other networks. For example a wireless network may operate proximate to other wireless networks. Concurrent transmission in each network may require the networks to take steps that increase signal quality at the cost of transmission speed. This may be acceptable when each network is transmitting data, but is wasteful when one network is simply transmitting unnecessary polling signals.
In addition, a polling protocol forces power-limited devices to keep their receivers on for extended periods of time, listening for polling signals even when no one in the network has any data to send. This can cause unnecessary drain on battery power, and can be particularly bad in devices in which the receiver circuitry represents a significant portion of the power consumption of the device. For example, ultrawide bandwidth devices typically have complex and power-hungry receivers that employ a large amount of signal processing because the transmission signals are below the noise floor. Such devices experience significant power drain any time their receiver is turned on.
It is therefore desirable to provide a system and method for data transmission in a communications network in which collisions can be minimized, but in which unnecessary transmissions by a network coordinator are also minimized.